For circuits which drive substantial inductive loads, such as fuel injector solenoids or ignition coils, typically some sort of inductive flyback protection circuit is required to prevent damage to the driver circuitry by large inductive flyback voltages. Such flyback circuitry limits the magnitude of the flyback voltages and also assists in dissipating the flyback energy to ensure proper rapid deactivation of an inductance.
Typically a zener diode has been utilized to limit flyback voltage and thereby protect the inductive load driver stage. U.S. Pat. No. 4,618,908 to Anttila illustrates one such zener diode protection circuit in which a zener diode in series with a resistor is connected between the collector of a Darlington driver stage and its base electrode. This configuration enables the use of a low power zener diode, but stills requires the use of a relatively expensive zener diode and resistor both of which are typically provided external to integrated circuit (IC) driver circuitry used to control the operative state of the Darlington driver stage. The zener diode and resistor typically have to be able to withstand high voltage transients at least for a short duration of time. Thus both components are generally discrete components provided external to a control IC. Also, such a configuration relies on the zener diode breakdown voltage for determining the maximum voltage which will be permitted to exist at the collector of the Darlington stage due to inductive flyback transient voltages. Zener diode voltages are only available in several discrete voltage values. Also, since these zener voltages cannot be very accurately controlled from device to device, this leads to a substantial variation in controlling the flyback voltage limit for different inductors. This may result in undesirable inherent unequal operation when such circuitry is used in conjunction with a plurality of inductive loads, such as the solenoid coils for a plurality of fuel injectors used in an engine.
Some circuits have utilized a resistor divider in conjunction with a zener diode to form a feedback path between the output collector of a power driver device and its base so as to form an overvoltage protection circuit to prevent excessive voltages from existing at the output electrode of the driver device. Sometimes the voltage divider is provided inside an integrated circuit which also includes the zener diode. However, in this configuration the maximum voltage which might be applied to the driver output terminal cannot be accurately controlled since it is a function of the zener diode breakdown voltage and it is known that there is a substantial tolerance with regard to the breakdown voltage of zener diodes. Other circuits have apparently utilized two external resistors in a voltage divider configuration while utilizing a zener diode inside an integrated circuit control circuit to accomplish a similar end result. This results in the use of two resistors external to the integrated circuit and therefore increases the parts count and cost of such a circuit. In addition, in such prior circuits there is still no way to have the integrated circuit itself adjust the maximum voltage level for voltages applied to the output electrode of the driver stage. This is the voltage level which can exist prior to the zener diode breaking down and providing a measure of protection for limiting the voltage applied to the driver stage.